First synthesis of blastidic acid, a component amino acid in an antibiotic, blasticidin S

Article abstract of DOI:10.1016/S0040-4039(01)00010-7

Blastidic acid, a component amino acid in an antibiotic, blasticidin S, was synthesized for the first time from α,γ-diaminobutyric acid. The synthesis was achieved through elongation of the carbon chain of the starting amino acid, and N-methylation and am

Full text of DOI:10.1016/S0040-4039(01)00010-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1753–1755
First synthesis of blastidic acid, a component amino acid in an
antibiotic, blasticidin S
Shinya Nomoto* and Akira Shimoyama
Department of Chemistry, University of Tsukuba, Tsukuba 305-8571, Japan
Received 29 November 2000; revised 20 December 2000; accepted 22 December 2000
Abstract—Blastidic acid, a component amino acid in an antibiotic, blasticidin S, was synthesized for the first time from
a,g-diaminobutyric acid. The synthesis was achieved through elongation of the carbon chain of the starting amino acid, and
N-methylation and amidination of an v-amino group. © 2001 Elsevier Science Ltd. All rights reserved.
Blasticidin S is an antibiotic produced by Streptomyces
griseochromogenes, and has been used extensively as an
excellent fungicide against Pricularia oryzae, a serious
cause of rice blast disease in Japan.1 The structure (1)
was chemically determined by Otake et al.2 and con-
firmed by X-ray analysis3 to be composed of a guani-
dino b-amino acid moiety, blastidic acid, and a
nucleoside moiety, cytosinine. Although several groups
have synthesized cytosinine4,5 and analogs of blasticidin
S,6–8 the synthesis of blastidic acid and total synthesis
of this antibiotic have not hitherto been achieved. In
this communication, we wish to report the first synthe-
amino group must be performed in suitable order. The
carbon chain elongation producing b-ornithine can be
effected by the modified Arndt–Eistert reaction.9 This
conversion involves activation of a carboxyl group in
an N-protected amino acid as a mixed anhydride,
which is so reactive as to be liable to form a lactam
with a guanidino or even a protected guanidino group.
Therefore, the amidination should be carried out
preferably after the Arndt–Eistert reaction. On the
other hand, in preliminary studies on N-methylation
reactions using Na-t-butoxycarbonyl-A2bu, methyl
iodide treatment of a tosylamino group in alkaline
solution was revealed to be the most suitable method,
introducing a methyl group in high yield without any
side reaction. Since the methylation requires several
hours under alkaline conditions, this procedure should
not precede the chain elongation in order to avoid a
possible racemization of A2bu. The Arndt–Eistert reac-
tion could not be carried out in the presence of a
tosylamino group, which is reactive enough toward a
mixed anhydride. Therefore, we decided to perform the
synthesis according to the following sequence of reac-
tions: chain elongation, Nv-tosylation, Nv-methylation
and amidination.
sis of
L
-blastidic acid from
L
-a,g-diaminobutyric acid
(A2bu).
First,
Na-t-butoxycarbonyl-Ng-benzyloxycarbonyl-
A2bu (2, Scheme 1) was prepared by t-butoxycarbony-
lation of Ng-benzyloxycarbonyl-A2bu derived from a
copper complex of A2bu. The Arndt–Eistert reaction
for 2 was conducted according to the conventional
method9 to give an oily b-ornithine derivative 3, which
was purified on a silica gel column to obtain the pure
methyl ester 3. The ester 3 was saponified to afford a
crystalline carboxylic acid 4, which was then hydro-
genated with a palladium–charcoal catalyst to remove
In the synthetic path to the target, elongation of the
carbon chain at the carboxyl end of A2bu, methylation
of an v-amino group and amidination of a secondary
Keywords: blasticidin S; blastidic acid; Arndt–Eistert reaction; amidi-
nation.
* Corresponding author. Fax: +81
0 298 55 7610; e-mail:
nomoto@staff.chem.tsukuba.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00010-7

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S. Nomoto, A. Shimoyama / Tetrahedron Letters 42 (2001) 1753–1755
the benzyloxycarbonyl group in 4, resulting in forma-
tion of a crystal of 5. After tosylation of the d-amino
group in 5, the resulting crystal of 6 was subjected to
N-methylation with 10 equivalents of methyl iodide in a
mixture (1:1) of 2 M NaOH aqueous solution and
dioxane for 5 hours. The resulting N-methylated
product was recrystallized from acetone–ether to give a
the t-butoxycarbonyl group and then hydrogenated in
the presence of palladium–charcoal in water for 20
hours to remove the nitro group. The deprotected
product was applied to an Amberlite IRC120 column
and eluted with 3 M HCl to afford the expected amino
acid dihydrochloride, which was recrystallized from
boiling ethanol containing a small amount of water to
1
1
pure crystal of 7, H NMR (270 MHz, CD3OD): l 2.70
give needles10 in 55% yield. In Fig. 1, the H and 13C
(s, 3H, Nd-CH3), 3.02 (m, 2H, d-CH2).
NMR spectra of the synthetic blastidic acid are shown
1
and the H NMR spectrum agreed well with that of the
A tosyl group in 7 was selectively removed with sodium
in liquid ammonia, and the deprotected product, with-
out being isolated, was subjected to nitroamidination
with O-methyl-N-nitroisourea in 2 M NaOH aqueous
solution for 5 hours. The reaction mixture was
extracted with ethyl acetate ten times to remove the
unreacted isourea, adjusted to pH 3 and extracted with
chloroform 30 times. It was difficult to extract the
amidinated product completely from the water layer
and also to separate the product from the unreacted
isourea, but the product in water, without being iso-
lated, could be deprotected as stated below. From the
chloroform layer, the desired product 8 was obtained in
25% yield as a white solid, 1H NMR (270 MHz,
CD3OD): l 3.04 (s, 3H, Nd-CH3), 3.50 (m, 2H, d-CH2).
This compound is a protected form of blastidic acid,
which can be used for peptide synthesis. NG-Nitro-Nb-
t-butoxycarbonylblastidic acid (8), thus obtained, was
treated for 1 hour with trifluoroacetic acid to remove
natural blastidic acid.11 From the above-stated water
layer that resulted from the amidination, blastidic acid
was also obtained in 14% yield after de-t-butoxycar-
bonylation with 6 M HCl and hydrogenation followed
by chromatographic purification using an Amberlite
column.
On the column chromatography, Nd-methyl-b-ornithine
(9) was recovered in 32% yield and was amidinated to
give blastidic acid as follows. We found in this study
that b,v-diamino acids form copper complexes, which
can be utilized for selective modification of their v-
amino groups. Thus, amino acid 9, obtained also from
7 on deprotection with 25% HBr at 110°C for 2 hours,
was converted to a copper complex in the usual manner
and subjected to reaction with O-methylisourea for 24
hours at a room temperature. The reaction mixture was
acidified with 6 M HCl, and H2S was bubbled into the
solution to remove copper ions. Chromatographic
Scheme 1. (a) Cl-COOEt, N-methylmorpholine, AcOEt, −15°C; (b) CH2N2, AcOEt–Et2O, −15°C; (c) C6H5COOAg, MeOH, 0°C;
(d) 2 M NaOH; (e) H2/5% Pd–C, MeOH; (f) Ts–Cl, 1 M NaOH; (g) MeI, 2 M NaOH–dioxane; (h) Na, liq. NH3, −70°C; (i)
O-methyl-N-nitroisourea, 2 M NaOH; (j) CF3COOH; (k) H2/5% Pd–C, MeOH; (l) 25% HBr, 110°C; (m) CuCO3·Cu(OH)2, H2O;
(n) O-methylisourea, 1 M NaOH; (o) H2S, 6 M HCl.

S. Nomoto, A. Shimoyama / Tetrahedron Letters 42 (2001) 1753–1755
1755
In conclusion, we have successfully constructed the
blastidic acid moiety, which had not been synthesized
ever since the structure was elucidated in 1965 although
there was a need to establish a synthetic route to the
total synthesis of blasticidin S and its analogs. The
synthetic strategy designed here may provide a general
method of obtaining structurally related amino acids
such as pseudoblastidone and stendomycidine, involved
in cytomycin and stendomycin, respectively. We have
also extended the scope of utility of diamino acid–cop-
per complexes in selective modification of amino
groups.
References
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hara, H. J. Antibiot. Ser. A. 1958, 11, 1–5.
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10. Synthetic blastidic acid: mp 203–205°C (dec.) (192–
192.5°C (dec.)11); [h]D18 +21° (c 1, H2O) ([h]D20 +25° (c 1,
H2O)11); retention time (min) on amino acid analysis
performed by conventional methods 145 (cf. Nd-Me-b-
Orn 120, b-Orn 116, Arg 111, Orn 91). Found: C, 31.68;
H, 7.06; Cl, 26.27; N, 20.83%. Calcd for C7H18Cl2N4O2·1/
4H2O: C, 31.65; H, 7.02; Cl, 26.69; N, 21.09%.
11. Otake, N.; Takeuchi, S.; Endo, T.; Yonehara, H. Agric.
Biol. Chem. 1966, 30, 132–141.
Figure 1. NMR spectra of natural and synthetic blastidic
acid.
purification of the mixture on an Amberlite column
afforded blastidic acid.
.
.